Acoustic wave devices are used in a variety of sensing applications. For example, acoustic delay lines can be used to sense environmental factors such as temperature and pressure, and can be used to measure variables such as mass, viscosity and density. Surface acoustic wave (SAW) delay lines have been employed to sense vapors in air by applying a surface film that preferentially binds the vapor to a surface of the SAW device. The surface film traps the vapor, causing a mass increase that changes the phase or amplitude of acoustic waves propagating along a piezoelectric substrate of the device. A surface transverse wave (STW) delay line may be used to sense the concentrations of chemicals in aqueous solutions by immobilizing antibodies on the surface of the STW device, whereupon the net surface mass increases as the antigen that is complementary to the antibodies is captured from the solutions. Again, phase shifts and/or changes in signal amplitude can be monitored to obtain a determination regarding the captured antigen.
Increasing the sensitivity of acoustic wave devices is an ever-present goal in the design of the various types of the devices. In practice, the maximum sensitivity is at least partially determined by the acoustic attenuation of wave propagation. That is, the same components of wave motion that cause an increase in sensitivity increase the attenuation that is experienced by the device. For example, a surface grating of fingers that is used to trap an acoustic wave to an STW device may be increased in thickness in order to improve sensitivity, but the thicker grating will increase the attenuation that results from a surface film or from a fluid to be analyzed.
Eventually a limit is reached at which changes to factors such as the thickness of a preferential surface film cannot be used to increase sensitivity. The limit cannot surpass that point at which increased film thickness will result in a film that causes wave attenuation that renders the device substantially unaffected by any chemical interaction. This limit at which monitoring the output of the device would make it appear that interaction of the device with a substance of interest will not increase wave attenuation is referred to herein as the "attenuation limit." While the attenuation limit has been described with reference to an acoustic wave device having a grating of wave-trapping fingers and a preferential surface film, the importance of the attenuation limit to maximizing the sensitivity of an acoustic wave device applies equally to all other types of acoustic wave devices and applies to the various sensing environments, e.g., the detection of a vapor within air and the determination of the viscosity of a liquid in which the device is immersed.
It is an object of the present invention to provide an acoustic wave device and a method of operating the device that allow further improvements to sensitivity.